The present invention relates to a series regulator circuit having a function for detecting voltage drops.
In the prior art, voltage instability that occurs when a circuit is activated may result in operational defects. To prevent such defects, a power-on reset circuit that outputs a reset signal for initializing the operation of each circuit when the voltage becomes stable has been proposed (for example, refer to Japanese Laid-Open Patent Publication No. 2000-31807, FIG. 2). The power-on reset circuit described in Japanese Laid-Open Patent Publication No. 2000-31807 includes a reference voltage generator for generating a reference voltage, a power supply voltage detector for generating activation voltage proportional to the power supply voltage, and a reset signal generator for comparing the reference voltage and the activation voltage to generate a reset signal. The reset signal generator outputs a first logic state until the power supply voltage reaches a predetermined level and a second logic state when the power supply voltage reaches the predetermined level. When outputting the second logic state, after a delay caused by a delay circuit, the reset circuit is inactivated and the second logic state is maintained.
A voltage drop detection circuit has been proposed as a circuit that switches signals in accordance with the level of a power supply voltage (for example, refer to National Semiconductor “LMS33460 3V Under Voltage Detector”, [online], searched on Jan. 26, 2006, Internet <URL: http://www.national.com/ds/LM/LMS33460.pdf>.) The voltage drop detection circuit detects voltage drops resulting from short-circuits or the like caused by one reason or another. This is effective for avoiding defects such as a voltage drop that may be caused by a sudden increase in load current. Referring to FIG. 3, a voltage drop detection circuit 100 includes a comparator 102 for comparing constant voltage Vref of a constant voltage source 101 with voltage decreased from input voltage VIN by an amount corresponding to the voltage drop caused by a resistor R. The comparator 102 has an output terminal connected to a gate terminal of an n-channel MOS transistor 103. Thus, the comparator 102 provides the MOS transistor 103 with a comparison result of the constant voltage Vref. As a result, the output voltage Vo changes in accordance with the voltage at the gate terminal of the MOS transistor 103. This enables the detection of a voltage drop.
In the prior art, a series regulator is used to ensure a constant output voltage even if the input voltage fluctuates (for example, refer to Japanese Laid-Open Patent Publication No. 9-265330, FIG. 1 and Japanese Laid-Open Patent Publication No. 2002-343874, FIG. 1). Japanese Laid-Open Patent Publication No. 9-265330 describes a reference potential generation circuit that uses a series regulator as a constant current source. The structure of the series regulator will now be described with reference to FIG. 4. A series regulator circuit 110 includes a constant current source 111, which is connected to a power supply voltage line VDD, and the collector terminal of a bipolar transistor 112. The emitter terminal of the transistor 112 is connected to a ground line GND via a resistor 113.
The series regulator circuit 110 includes an n-channel MOS transistor 114, which has a drain terminal connected to a power supply voltage line VDD. The MOS transistor 114 has a source terminal connected to the ground line GND via resistors 115 and 116 and the voltage here serves as output voltage Vout. Furthermore, the gate terminal of the MOS transistor 114 is connected to a connection node between the constant current source 111 and the collector terminal of the transistor 112. A connection node of resistors 115 and 116 is connected to the base terminal of the transistor 112.
The output voltage Vout of the series regulator circuit 110 is in accordance with a load current. When an increase in the load current decreases the output voltage, the base voltage Vbg applied to the base terminal of the transistor 112 decreases and accordingly reduces the collector current. In this case, the voltage at the collector terminal, or the voltage at the gate terminal of the MOS transistor 114 increases. This lowers the resistance between the drain and source of the MOS transistor 114 and increases the output voltage Vout. That is, the resistance of the MOS transistor 114 connected in series to the resistors 115 and 116 decreases so as to keep the output voltage Vout substantially constant.
Japanese Laid-Open Patent Publication No. 2002-343874 describes a series regulator circuit having another structure. FIG. 5 shows the structure of the series regulator circuit 120, which includes a constant voltage source 121, an operational amplifier 122, and an n-channel MOS transistor 123. The MOS transistor 123 has a drain terminal connected to power supply voltage line VDD, a source terminal connected to a ground line GND via resistors 124 and 125, and a gate terminal connected to an output terminal of the operational amplifier 122. When the voltage at the connection node of the resistors 124 and 125 decreases, a constant voltage Vref of the constant voltage source 121 changes the gate voltage of the MOS transistor 123 to increase the output voltage Vout. On the other hand, when the voltage at the connection node of the resistors 124 and 125 increases, the constant voltage Vref changes the gate voltage of the MOS transistor 123 to decrease the output voltage Vout. This keeps the output voltage Vout substantially constant.
The purpose of the above-described voltage drop detection circuit 100 and series regulator circuits 110 and 120 is to stabilize circuit operations when the output voltage Vout fluctuates. Accordingly, these circuits may be used in combination to form an integrated circuit.
As an example, FIG. 6 shows a series regulator circuit 150 that functions to detect voltage drops. The series regulator circuit 150 is formed by combining the voltage drop detection circuit 100 shown in FIG. 3 and the series regulator circuit 110 shown in FIG. 4. More specifically, the series regulator circuit 150 includes a constant voltage source 101 and a comparator 102, which compares the constant voltage Vref of the constant voltage source 101 with the output voltage Vout of the series regulator circuit 110. Accordingly, when the output voltage Vout becomes less than the constant voltage Vout, the series regulator circuit 150 outputs a step down voltage detection Vud from the comparator 102.
A series regulator circuit 160 shown in FIG. 7 may also be used to detect voltage drops. The series regulator circuit 160 is formed by combining the voltage drop detection circuit 100 shown in FIG. 3 and the series regulator circuit 120 shown in FIG. 5. More specifically, the series regulator circuit 160 supplies the constant voltage of a constant voltage source 121 to one of the input terminals of an operational amplifier 122 and one of the input terminals of a comparator 102. The other input terminal of the operational amplifier 122 is connected to a connection node between resistors 162 and 125, and the output terminal of the operational amplifier 122 is connected to the gate terminal of an n-MOS transistor 123. The source terminal of the MOS transistor 123 has the output voltage Vout and is connected to ground GND via resistors 161, 162, and 125. Further, the other input terminal of the comparator 102 is connected to a connection node of the resistors 161 and 162.
In the series regulator circuits 150 and 160, like or same reference numerals are given to those components that are the same as the corresponding components in the voltage drop detection circuit 100 and the series regulator circuits 110 and 120.
The consumed current of the series regulator circuit 150 shown in FIG. 6 in a static state is represented by IDDQ1. In this case, the consumed current IDDQ1 is the total value of the current Ip flowing through the constant current source 111 and transistor 112, the current IB flowing through the MOS transistor 114, the current IR consumed by the constant voltage source 101, and the current IC consumed by the comparator 102.
The consumed current of the series regulator circuit 160 shown in FIG. 7 in a static state is represented by IDDQ2. In this case, the consumed current IDDQ2 is the total value of the current IR consumed by the constant voltage source 121, the current IOP consumed by the operational amplifier 122, the current IB flowing through the MOS transistor 123, and the current IC consumed by the comparator 102.
Such series regulator circuits can be reduced in size by integrating circuits. However, the consumed current of these circuits must further be reduced.